Prismatic lithium secondary battery having a porous heat resistant layer

ABSTRACT

A prismatic lithium secondary battery includes: a prismatic battery can having a bottom, a side wall, and an open top; an electrode assembly; a non-aqueous electrolyte; and a sealing plate covering the open top of the battery can that accommodates the electrode assembly and the non-aqueous electrolyte. The electrode assembly includes: a positive electrode; a negative electrode; and a porous heat-resistant layer and a separator that are interposed between the positive and negative electrodes. The side wall of the battery can has two rectangular main flat portions that are opposed to each other, and the thickness A of the porous heat-resistant layer and the thickness B of each of the main flat portions of the side wall satisfy the relation: 0.003≦A/B≦0.05.

FIELD OF THE INVENTION

The present invention relates to a prismatic lithium secondary batterythat is excellent in both safety and battery characteristics.

BACKGROUND OF THE INVENTION

Lithium secondary batteries have received attention as high-capacitypower sources for portable and other appliances. Further, lithiumsecondary batteries have recently been receiving attention ashigh-output power sources for electric vehicles and the like. Chemicalbatteries such as lithium secondary batteries usually have a separatorthat electrically insulates a positive electrode from a negativeelectrode and holds an electrolyte. In the case of a lithium secondarybattery, a micro-porous film made of polyolefin (e.g., polyethylene,polypropylene, etc.) is mainly used as the separator. The electrodeassembly of a prismatic lithium secondary battery is produced by windingthe positive electrode, the negative electrode, and the separatorinterposed between the two electrodes such that the wound assembly issubstantially elliptic in cross-section.

However, when a lithium secondary battery is stored in an environment atextremely high temperatures for an extended period of time, itsseparator made of a micro-porous film tends to shrink. If the separatorshrinks, then the positive electrode and the negative electrode mayphysically come into contact with each other to cause an internalshort-circuit. In view of the recent tendency of separators becomingthinner with an increase in lithium secondary battery capacity,preventing an internal short-circuit becomes particularly important.Once an internal short-circuit occurs, the short-circuit may expand dueto Joule's heat generated by the short-circuit current, therebyresulting in overheating of the battery.

Thus, in the event of an internal short-circuit, in order to suppresssuch expansion of the short-circuit, Japanese Laid-Open PatentPublication No. Hei 7-220759 proposes forming a porous heat-resistantlayer that contains an inorganic filler (solid fine particles) and abinder on an electrode active material layer. Alumina, silica, or thelike is used as the inorganic filler. The inorganic filler is filled inthe porous heat-resistant layer, and the filler particles are bonded toone another with a relatively small amount of a binder. Since the porousheat-resistant layer is resistant to shrinking even at hightemperatures, it has the function of suppressing the overheating of thebattery in the event of an internal short-circuit.

Recently, in the field of the power source for portable appliances,there is an increasing need for rapid charge, and rapid charge requirescharging at a high rate (e.g., 1 hour-rate or less). In the case of ahigh-rate charge, the electrode plate expands and contractssignificantly during charge/discharge and a large amount of gas isproduced, compared with a low-rate charge (e.g., 1.5 hour-rate or more).Therefore, the electrode assembly is distorted, and the porousheat-resistant layer may break since the amount of the binder containedin the porous heat-resistant layer is relatively small and the bondingbetween filler particles is weak. In such cases, the function of theporous heat-resistant layer of suppressing the overheating of thebattery in the event of an internal short-circuit is impaired.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the present invention to prevent a breakageof the porous heat-resistant layer and provide a prismatic lithiumsecondary battery that is excellent in both safety and batterycharacteristics.

The present invention relates to a prismatic lithium secondary batteryincluding: a prismatic battery can having a bottom, a side wall, and anopen top; an electrode assembly; a non-aqueous electrolyte; and asealing plate covering the open top of the battery can that accommodatesthe electrode assembly and the non-aqueous electrolyte. The side wall ofthe prismatic battery can has two rectangular main flat portions thatare opposed to each other. The electrode assembly includes: a positiveelectrode; a negative electrode; and a porous heat-resistant layer and aseparator that are interposed between the positive and negativeelectrodes. The thickness A of the porous heat-resistant layer and thethickness B of each of the main flat portions of the side wall satisfythe relation:0.003≦A/B≦0.05.

The open top of the prismatic battery can is substantially rectangular,and the longer sides of this substantial rectangle correspond to themain flat portions. That is, the main flat portions correspond to thewider surfaces of the side wall.

It is preferred that the thickness A of the porous heat-resistant layerbe 2 to 10 μm and that the thickness B of each of the main flat portionsof the side wall be 160 to 1000 μm. Further, it is preferred that0.005≦A/B≦0.03.

The electrode assembly may be of the wound type. For example, thepositive electrode is a strip-like positive electrode that comprises apositive electrode core member and a positive electrode active materiallayer carried on each side of the positive electrode core member, andthe negative electrode is a strip-like negative electrode that comprisesa negative electrode core member and a negative electrode activematerial layer carried on each side of the negative electrode coremember. These strip-like positive and negative electrodes are woundtogether with the porous heat-resistant layer and the separatorinterposed between the positive and negative electrodes. In this case,it is preferable that the porous heat-resistant layer be carried on asurface of at least one of the two active material layers that areformed on both sides of the core member of at least one of the positiveelectrode and the negative electrode.

The electrode assembly may be of the stacked type. For example, theelectrode assembly may be composed of at least one sheet-like positiveelectrode and at least one sheet-like negative electrode that arestacked with a porous heat-resistant layer and a separator interposedtherebetween. In this case, the outermost electrodes preferably have anactive material layer only on one side (inner side) of the core member.

The porous heat-resistant layer preferably comprises an insulatingfiller.

The insulating filler preferably comprises an inorganic oxide.

When the battery is charged at a high rate, a large distortion isapplied to the electrode assembly. However, if the main flat portions ofside wall of the battery can pushes back the electrode assembly by asufficient force, the porous heat-resistant layer does not break. Inthis case, the porous heat-resistant layer can retain its shape probablybecause the porous heat-resistant layer is pressed against the activematerial layer of the positive or negative electrode.

It should be noted that the porous heat-resistant layer is also requiredto have the function of holding an electrolyte between the positiveelectrode and the negative electrode. Hence, if the porousheat-resistant layer is excessively pressed against the active materiallayer, the electrolyte becomes locally scarce in the electrode assembly,thereby resulting in degradation of battery characteristics.

The present invention is based on the above two findings. The presentinvention proposes controlling the force of the main flat portions ofside wall of the battery can which pushes back the porous heat-resistantlayer in an appropriate range, depending on the thickness of the porousheat-resistant layer. Accordingly, it is possible to prevent the porousheat-resistant layer from breaking and ensure safety in the event ofinternal short-circuits. Further, it is also possible to realizeexcellent battery characteristics.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a sectional schematic view of a part of a prismatic lithiumsecondary battery in accordance with the present invention; and

FIG. 2 is a longitudinal sectional view of a prismatic lithium secondarybattery in accordance with an example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic view of a part of a prismatic lithium secondarybattery in accordance with the present invention.

A positive electrode 13 has a strip-like positive electrode core member11 and a positive electrode active material layer 12 carried on eachside of the core member 11. A negative electrode 16 has a strip-likenegative electrode core member 14 and a negative electrode activematerial layer 15 carried on each side of the core member 14. A porousheat-resistant layer 18 is carried on the surface of each negativeelectrode active material layer 15. The porous heat-resistant layer 18has the function of preventing the expansion of an internalshort-circuit. The positive electrode 13 and the negative electrode 16are wound together with a strip-like separator 17 and the porousheat-resistant layer 18 interposed between the two electrodes, to forman electrode assembly. An exposed part 14 a of the negative electrodecore member is positioned in the outermost turn of the electrodeassembly. The electrode assembly is housed in a prismatic battery can19.

In the present invention, the thickness A of the porous heat-resistantlayer and the thickness B of the main flat portions of side wall of thebattery can satisfy the relation: 0.003≦A/B≦0.05. The porousheat-resistant layer has the function of ensuring short-circuitresistance (first function) and the function of holding an electrolyte(second function). If the force of the main flat portions of side wallof the battery can which pushes back the electrode assembly isinsufficient, the porous heat-resistant layer is likely to break duringa high-rate charge, so that the first function is impaired. On the otherhand, if the force of the main flat portions of side wall of the batterycan which pushes back the electrode assembly is excessive, the porousheat-resistant layer is strongly compressed, so that it cannot hold asufficient amount of an electrolyte. As a result, the second function isimpaired.

When A/B≦0.003, the thickness A of the porous heat-resistant layer istoo thin relative to the thickness B of the main flat portions of sidewall of the battery can. If the porous heat-resistant layer is thin, theamount of the electrolyte held therein is small. In addition, due to thelarge pressure exerted on the electrode assembly from the main flatportions of side wall of the battery can, the electrolyte tends to besqueezed out of the porous heat-resistant layer. As a result, theelectrolyte becomes locally scarce in the electrode assembly, therebyresulting in degradation of battery characteristics. In terms ofoptimizing the balance between the first function and the secondfunction, desirably 0.005≦A/B, and more desirably 0.01≦A/B.

When 0.05<A/B, the thickness A of the porous heat-resistant layer is toothick relative to the thickness B of the main flat portions of side wallof the battery can. If the porous heat-resistant layer is thick, itsflexibility deteriorates, so that the porous heat-resistant layerbecomes brittle. Hence, when the electrode assembly deforms during ahigh-rate charge, the porous heat-resistant layer is likely to break. Inaddition, since the force of the main flat portions of side wall of thebattery can which pushes back the electrode assembly is insufficient,the porous heat-resistant layer is not sufficiently supported. Thus, theporous heat-resistant layer easily breaks and the short-circuitresistance of the battery degrades. In terms of optimizing the balancebetween the first function and the second function, desirably A/B≦0.03,and more desirably A/B≦0.025. From the above, preferably 0.005≦A/B≦0.03,and more preferably 0.01≦A/B≦0.025.

The thickness A of the porous heat-resistant layer is preferably 2 to 10μm, and more preferably 3 to 8 μm. If the thickness A is too thin, thefunction of improving short-circuit resistance or the function ofholding the electrolyte may become insufficient. If the thickness A istoo thick, there is an excessively large distance between the positiveelectrode and the negative electrode, which may result in degradation ofthe output characteristics.

The thickness B of the main flat portions of side wall of the batterycan is preferably 160 to 1000 μm, and more preferably 200 to 500 μm. Ifthe thickness B is too thin, it may be difficult to form the batterycan. If the thickness B is too thick, it becomes difficult to heightenthe energy density of the battery.

A micro-porous film is preferably used as the separator. The material ofthe micro-porous film is preferably polyolefin, and the polyolefin ispreferably polyethylene, polypropylene, or the like. A micro-porous filmcomprising both polyethylene and polypropylene may also be used. Thethickness of the micro-porous film is preferably 8 to 20 μm in terms ofmaintaining a high capacity design.

The porous heat-resistant layer may be formed on only the surface of thepositive electrode active material layer or only the surface of thenegative electrode active material layer. Alternatively, it may beformed on the surface of the positive electrode active material layerand the surface of the negative electrode active material layer.However, in order to avoid an internal short-circuit in a reliablemanner, the porous heat-resistant layer is desirably formed on thesurface of the negative electrode active material layer that is designedto have a larger area than that of the positive electrode activematerial layer. Also, the porous heat-resistant layer may be formed onthe active material layer on one side of the core member or may beformed on the active material layers on both sides of the core member.Further, the porous heat-resistant layer is desirably adhered to thesurface of the active material layer.

The porous heat-resistant layer may be in the form of an independentsheet. However, since the porous heat-resistant layer in sheet form doesnot have a high mechanical strength, it may be difficult to handle.Also, the porous heat-resistant layer may be formed on the surface ofthe separator. However, since the separator shrinks at hightemperatures, close attention must be given to manufacturing conditionsof the porous heat-resistant layer. In terms of eliminating suchconcern, it is also desirable that the porous heat-resistant layer beformed on the surface of the positive electrode active material layer orthe surface of the negative electrode active material layer. The porousheat-resistant layer has a large number of pores. Thus, even if it isformed on the surface of the positive electrode active material layer,negative electrode active material layer or separator, it does notinterfere with the movement of lithium ions. Porous heat-resistantlayers having the same composition or a different composition may belaminated.

The porous heat-resistant layer preferably contains an insulating fillerand a binder. Such a porous heat-resistant layer is formed by applying araw material paste, containing an insulating filler and a small amountof a binder, onto the surface of the electrode active material layerwith a doctor blade or a die coater and drying it. The raw materialpaste is prepared by mixing an insulating filler, a binder, and a liquidcomponent, for example, with a double-arm kneader.

Also, the porous heat-resistant layer may be a film formed of fibers ofa highly heat-resistant resin. The highly heat-resistant resin ispreferably aramid, polyamide imide, etc. However, the porousheat-resistant layer comprising an insulating filler and a binder has ahigher structural strength, due to the action of the binder, than thefilm formed of fibers of a highly heat-resistant resin and ispreferable.

The insulating filler may comprise fibers or beads of the highlyheat-resistant resin, but it preferably comprises an inorganic oxide.Since inorganic oxides are hard, they can maintain the distance betweenthe positive electrode and the negative electrode in an appropriaterange even if the electrode expands due to charge/discharge. Amonginorganic oxides, for example, alumina, silica, magnesia, titania, andzirconia are particularly preferable, because they are electrochemicallyhighly stable in the operating environment of lithium secondarybatteries. They may be used singly or in combination of two or more ofthem. Also, the insulating filler may be a highly heat-resistant resinsuch as aramid or polyamide imide. It is also possible to use acombination of an inorganic oxide and a highly heat-resistant resin.

In the porous heat-resistant layer comprising such an insulating fillerand a binder, the amount of the binder is preferably 1 to 10 parts byweight, more preferably 2 to 8 parts by weight, per 100 parts by weightof the insulating filler, in order to maintain its mechanical strengthand its ionic conductivity. Most binders and thickeners inherently swellwith a non-aqueous electrolyte. Thus, if the amount of the binderexceeds 10 parts by weight, the binder swells excessively to close thepores of the porous heat-resistant layer, so that the ionic conductivitymay lower and the battery reaction may be impeded. On the other hand, ifthe amount of the binder is less than 1 part by weight, the mechanicalstrength of the porous heat-resistant layer may degrade.

The binder used in the porous heat-resistant layer is not particularlylimited, but polyvinylidene fluoride (hereinafter referred to as PVDF),polytetrafluoroethylene (hereinafter referred to as PTFE), andpolyacrylic acid-type rubber particles (e.g., BM-500B (trade name)available from Zeon Corporation), for example, are preferred. It ispreferred to use PTFE or BM-500B in combination with a thickener. Thethickener is not particularly limited, but carboxymethyl cellulose(hereinafter referred to as CMC), polyethylene oxide (hereinafterreferred to as PEO), and modified acrylonitrile rubber (e.g., BM-720H(trade name) available from Zeon Corporation), for example, arepreferred.

The porosity of the porous heat-resistant layer comprising theinsulating filler and the binder is preferably 40 to 80%, morepreferably 45 to 65%, in order to maintain its mechanical strength andsecure the ionic conductivity. When the porous heat-resistant layer witha porosity of 40 to 80% is impregnated with a suitable amount ofelectrolyte, the electrode assembly swells to a suitable extent. As aresult, the swollen electrode assembly presses the inner side wall ofthe battery can to a suitable extent. When this effect obtained from theporosity of 40 to 80% is synergistically combined with the effect ofoptimization of the A/B ratio, a battery that is particularly excellentin the balance between the first function and the second function can beobtained.

It should be noted that the porosity of the porous heat-resistant layercan be controlled by changing the median diameter of the insulatingfiller, the amount of the binder, and the drying conditions of the rawmaterial paste. For example, increasing the drying temperature or theflow rate of hot air for the drying results in a relative increase inporosity. The porosity can be calculated from, for example, thethickness of the porous heat-resistant layer, the amounts of theinsulating filler and the binder, and the true specific gravities of theinsulating filler and the binder. The thickness of the porousheat-resistant layer can be determined by taking an SEM photo of severalcross-sections (for example, 10 cross-sections) of an electrode andaveraging the thicknesses in the several cross-sections. Also, theporosity can be determined with a mercury porosimeter.

The positive electrode includes, for example, a positive electrode coremember and a positive electrode active material layer carried on eachside thereof. The positive electrode core member is, for example, in theform of a strip suitable for winding and comprises Al, an Al alloy, orthe like. The positive electrode active material layer contains apositive electrode active material as an essential component and maycontain optional components such as a conductive agent and a binder.These materials are not particularly limited, but a preferable positiveelectrode active material is a lithium-containing transition metaloxide. Among lithium-containing transition metal oxides, lithiumcobaltate, modified lithium cobaltate, lithium nickelate, modifiedlithium nickelate, lithium manganate and modified lithium manganate arepreferred, for example.

The negative electrode includes, for example, a negative electrode coremember and a negative electrode active material layer carried on eachside thereof. The negative electrode core member is, for example, in theform of a strip suitable for winding and comprises Cu, a Cu alloy, orthe like. The negative electrode active material layer contains anegative electrode active material as an essential component and maycontain optional components such as a conductive agent and a binder.These materials are not particularly limited, but preferable negativeelectrode active materials include various natural graphites, variousartificial graphites, silicon-containing composite materials such assilicide, lithium metal, and various alloy materials.

Exemplary binders for the positive or negative electrode include PTFE,PVDF, and styrene butadiene rubber. Exemplary conductive agents includeacetylene black, ketjen black (registered trademark), and variousgraphites.

The non-aqueous electrolyte preferably comprises a non-aqueous solventdissolving a lithium salt. The lithium salt is not particularly limited,but for example, LiPF₆ and LiBF₄ are preferred. Such lithium salts maybe used singly or in combination of two or more of them. The non-aqueoussolvent is not particularly limited, but preferable examples includeethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate(DEC), and ethyl methyl carbonate (EMC). Such non-aqueous solvents maybe used singly or in combination of two or more of them.

The material of the battery can must be electrochemically stable in theoperating voltage range of lithium secondary batteries. For example,aluminum, iron, or stainless steel is preferably used. Also, the batterycan may be plated with nickel or tin.

The present invention is hereinafter described more specifically by wayof Examples.

EXAMPLE 1

(Battery 1)

(i) Preparation of Positive Electrode

A positive electrode mixture paste was prepared by stirring 3 kg oflithium cobaltate, 1 kg of PVDF#1320 available from KUREHA CORPORATION(N-methyl-2-pyrrolidone (hereinafter referred to as NMP) solutioncontaining 12% by weight of PVDF), 90 g of acetylene black, and asuitable amount of NMP with a double-arm kneader. This paste was appliedonto both sides of a positive electrode core member comprising a15-μm-thick aluminum foil, dried, and rolled, to form a positiveelectrode with positive electrode active material layers. This positiveelectrode had a total thickness of 130 μm. The positive electrode wascut to a strip with a width of 43 mm.

(ii) Preparation of Negative Electrode

A negative electrode mixture paste was prepared by stirring 3 kg ofartificial graphite, 75 g of BM-400B available from Zeon Corporation(aqueous dispersion containing 40% by weight of modified styrenebutadiene rubber), 30 g of CMC, and a suitable amount of water with adouble-arm kneader. This paste was applied onto both sides of a negativeelectrode core member comprising a 10-μm-thick copper foil, dried, androlled to form a negative electrode with negative electrode activematerial layers. This negative electrode had a total thickness of 140μm. The negative electrode was cut to a strip with a width of 45 mm.

(iii) Formation of Porous Heat-Resistant Layer

A raw material paste was prepared by stirring 970 g of alumina with amedian diameter of 0.3 μm (insulating filler), 375 g of BM-720Havailable from Zeon Corporation (NMP solution containing 8% by weight ofmodified polyacrylonitrile rubber (binder)), and a suitable amount ofNMP with a double-arm kneader. This raw material paste was applied ontothe surfaces of the negative electrode active material layers and driedunder reduced pressure at 120° C. for 10 hours, to form 0.5-μm-thickporous heat-resistant layers.

The porosity of each porous heat-resistant layer was 48%. The porositywas calculated from: the thickness of the porous heat-resistant layerdetermined by taking an SEM photo of a cross-section thereof; the amountof alumina in the porous heat-resistant layer of a given area obtainedby X-ray fluorescence analysis; the true specific gravities of aluminaand the binder; and the weight ratio between alumina and the binder.

(iv) Preparation of Non-Aqueous Electrolyte

LiPF₆ was dissolved at a concentration of 1 mol/liter in a solventmixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethylmethyl carbonate (EMC) in a volume ratio of 1:1:1, and this solution wasmixed with 3% by weight of vinylene carbonate, to prepare a non-aqueouselectrolyte.

(v) Fabrication of Battery

An explanation is made with reference to FIG. 2, in which elements otherthan the electrode assembly are cross-sectional.

An electrode assembly 21 with a substantially elliptic cross-section wasfabricated by winding the positive electrode and the negative electrodewith the porous heat-resistant layers formed on both sides thereof, witha separator comprising a 20-μm-thick polyethylene micro-porous film(A089 (trade name) available from Celgard K. K.) interposedtherebetween.

The electrode assembly 21 was placed in an aluminum prismatic batterycan 20. The battery can 20 has a bottom 20 a and a side wall 20 b. Thetop of the battery can 20 is open and substantially rectangular. Theside wall 20 b has main flat portions, and the thickness of each mainflat portion was 80 μm.

Thereafter, an insulator 24 was mounted on the electrode assembly 21 toprevent a short-circuit between the battery can 20 and a positiveelectrode lead 22 or a negative electrode lead 23. A rectangular sealingplate 25 was fitted to the open top of the battery can 20. The sealingplate 25 had, at the center thereof, a negative electrode terminal 27around which an insulating gasket 26 was fitted. The negative electrodelead 23 was connected to the negative electrode terminal 27, and thepositive electrode lead 22 was connected to the lower face of thesealing plate 25. The open top of the battery can 20 was sealed bylaser-welding the open edge and the sealing plate 25. Then, 2.5 g of thenon-aqueous electrolyte was injected into the battery can 20 from theinjection hole of the sealing plate 25. Lastly, the injection hole wasclosed with a sealing stopper 29 by welding. This completed a prismaticlithium secondary battery with a height of 50 mm, a width of 34 mm, aninner space thickness of approximately 5.2 mm, and a design capacity of850 mAh.

(Batteries 2 to 6)

Prismatic lithium secondary batteries 2, 3, 4, 5, and 6 were produced inthe same manner as the battery 1, except that the thickness of the mainflat portions of side wall of the battery can was changed to 160 μm, 300μm, 600 μm, 1000 μm, and 1500 μm, respectively.

(Batteries 7 to 12)

Prismatic lithium secondary batteries 7, 8, 9, 10, 11, and 12 wereproduced in the same manner as the batteries 1, 2, 3, 4, 5, and 6,respectively, except that the thickness of the porous heat-resistantlayer was changed to 1 μm.

(Battery 13 to 18)

Prismatic lithium secondary batteries 13, 14, 15, 16, 17, and 18 wereproduced in the same manner as the batteries 1, 2, 3, 4, 5, and 6,respectively, except that the thickness of the porous heat-resistantlayer was changed to 2 μm.

(Batteries 19 to 24)

Prismatic lithium secondary batteries 19, 20, 21, 22, 23, and 24 wereproduced in the same manner as the batteries 1, 2, 3, 4, 5, and 6,respectively, except that the thickness of the porous heat-resistantlayer was changed to 3 μm.

(Batteries 25 to 32)

(i) Batteries 25 to 27, 29, 30, and 32

Prismatic lithium secondary batteries 25, 26, 27, 29, 30, and 32 wereproduced in the same manner as the batteries 1, 2, 3, 4, 5, and 6,except that the thickness of the porous heat-resistant layer was changedto 4 μm.

(ii) Battery 28

A prismatic lithium secondary battery 28 was produced in the same manneras the battery 1, except that the thickness of the main flat portions ofside wall of the battery can was changed to 400 μm and that thethickness of the porous heat-resistant layer was changed to 4 μm.

(iii) Battery 31

A prismatic lithium secondary battery 31 was produced in the same manneras the battery 1, except that the thickness of the main flat portions ofside wall of the battery can was changed to 1200 μm and that thethickness of the porous heat-resistant layer was changed to 4 μm.

(Batteries 33 to 40)

Prismatic lithium secondary batteries 33, 34, 35, 36, 37, 38, 39, and 40were produced in the same manner as the batteries 25, 26, 27, 28, 29,30, 31, and 32, respectively, except that the thickness of the porousheat-resistant layer was changed to 7 μm.

(Batteries 41 to 48)

Prismatic lithium secondary batteries 41, 42, 42, 44, 45, 46, 47, and 48were produced in the same manner as the batteries 25, 26, 27, 28, 29,30, 31, and 32, respectively, except that the thickness of the porousheat-resistant layer was changed to 10 μm.

(Batteries 49 to 54)

Prismatic lithium secondary batteries 49, 50, 51, 52, 53, and 54 wereproduced in the same manner as the batteries 1, 2, 3, 4, 5, and 6,respectively, except that the thickness of the porous heat-resistantlayer was changed to 20 μm.

In the batteries 2 to 54, the porosity of the porous heat-resistantlayer was 46 to 49%.

[Evaluation]

The respective batteries were preliminarily charged and discharged twiceand then stored in an environment at 45° C. for 7 days. Thereafter, theywere evaluated in the following manner. Table 1 shows the thickness A ofthe porous heat-resistant layer, the thickness B of the main flatportions of side wall of the battery can, and evaluation results.

(Nail Penetration Test)

The respective batteries were charged at a charge current of 850 mA to acut-off voltage of 4.35 V or 4.45 V. In an environment at 20° C., a2.7-mm-diameter iron nail was driven into the side wall of each chargedbattery at a speed of 5 mm/sec, and the battery temperature was measuredwith a thermocouple fitted to the side wall of the battery. Thetemperature after 90 seconds was measured.

(Cycle Life Test)

In the 20° C. environment, the batteries were charged and dischargedunder the following condition (1) or (2) for 500 cycles. The percentageof the discharge capacity at the 500th cycle (capacity retention rate)relative to the initial discharge capacity was obtained.

Condition (1)

Constant current charge: charge current 850 mA/end of charge voltage 4.2V

Constant voltage charge: charge voltage 4.2 V/end of charge current 100mA

Constant current discharge: discharge current 850 mA/end of dischargevoltage 3 V

Condition (2)

Constant current charge: charge current 850 mA/end harge voltage 4.2 V

Constant voltage charge: charge voltage 4.2 V/end harge current 100 mA

Constant current discharge: discharge current 1700 mA/end of dischargevoltage 3 V

TABLE 1 Nail penetration Capacity retention Thickness A of Thickness Btest rate (%) porous heat- of main flat Battery temperature 850 mA 1700mA resistant layer portion after 90 sec (° C.) discharge dischargeBattery (μm) (μm) A/B 4.35 V 4.45 V 1 C 2 C 1 0.5 80 0.0063 93 112 76 572 0.5 160 0.0031 95 105 78 54 3 0.5 300 0.0017 94 103 63 42 4 0.5 6000.0008 96 103 59 42 5 0.5 1000 0.0005 98 104 56 39 6 0.5 1500 0.0003 90106 55 38 7 1 80 0.0125 94 114 75 56 8 1 160 0.0063 93 102 77 57 9 1 3000.0033 90 104 80 55 10 1 600 0.0017 96 102 62 40 11 1 1000 0.0010 89 10564 43 12 1 1500 0.0007 87 106 60 37 13 2 80 0.0250 97 116 80 66 14 2 1600.0125 92 101 76 64 15 2 300 0.0067 90 100 81 66 16 2 600 0.0033 93 10478 52 17 2 1000 0.0020 94 104 61 42 18 2 1500 0.0013 92 105 63 38 19 380 0.0375 96 117 75 51 20 3 160 0.0188 95 106 75 66 21 3 300 0.0100 91107 78 68 22 3 600 0.0050 87 103 81 63 23 3 1000 0.0030 91 104 74 56 243 1500 0.0020 94 105 64 46 25 4 80 0.0500 96 116 78 61 26 4 160 0.025090 102 80 62 27 4 300 0.0133 88 102 83 65 28 4 400 0.0100 85 105 77 6429 4 600 0.0067 91 103 81 63 30 4 1000 0.0040 83 100 75 53 31 4 12000.0033 90 102 76 55 32 4 1500 0.0027 89 101 58 42 33 7 80 0.0875 123 13378 66 34 7 160 0.0438 87 118 81 66 35 7 300 0.0233 91 106 80 66 36 7 4000.0175 86 106 76 62 37 7 600 0.0117 93 103 78 64 38 7 1000 0.0070 88 10576 66 39 7 1200 0.0058 84 105 74 54 40 7 1500 0.0047 86 101 74 56 41 1080 0.1250 126 140 78 69 42 10 160 0.0625 118 131 79 68 43 10 300 0.033396 115 76 64 44 10 400 0.0250 94 104 80 64 45 10 600 0.0167 89 102 82 6546 10 1000 0.0100 86 106 79 64 47 10 1200 0.0083 88 105 83 55 48 10 15000.0067 91 106 80 53 49 20 80 0.2500 130 142 82 56 50 20 160 0.1250 127136 78 56 51 20 300 0.0667 124 135 84 52 52 20 600 0.0333 90 122 83 5853 20 1000 0.0200 87 115 78 56 54 20 1500 0.0133 85 114 81 56

In the case of the batteries 3 to 6, 10 to 12, 17, 18, 24, and 32 withthe A/B ratios (the ratio of the thickness A (μm) of the porousheat-resistant layer to the thickness B (μm) of the main flat portionsof side wall of the battery can) of less than 0.003, the cycle lifecharacteristic was remarkably low. This result is related to the factthat the thickness of the porous heat-resistant layer is thin relativeto the main flat portions of the battery can. If the porousheat-resistant layer is thin, the amount of the electrolyte that it canhold is small and, in addition, the electrolyte is likely to be squeezedout due to the pressure from the main flat portions of side wall of thebattery can. Therefore, the electrolyte inside the electrode assembly isbelieved to become scarce.

On the other hand, in the case of the batteries 33, 41, 42, 49, 50, and51 with the A/B ratios of more than 0.05, the overheating on the nailpenetration test was remarkable. When these batteries were disassembled,it was found that the porous heat-resistant layer was separated not onlyat the nail penetration location but also at many other areas. Thisresult is related to the fact that the thickness of the porousheat-resistant layer is thick relative to the battery can. When theporous heat-resistant layer is thick, it becomes brittle, so it easilybreaks due to deformation of the electrode assembly during a high-ratecharge. Further, since the side wall of the battery can is thin, theforce thereof which pushes back the electrode assembly is also weak.Probably for this reason, the porous heat-resistant layer became broken.

With respect to the batteries 1 to 12, the cycle life characteristic wasremarkably low in the harsh charge/discharge condition (2) of 1700 mAdischarge, regardless of the thickness of the main flat portions of sidewall of the battery can. This indicates that when the porousheat-resistant layer has a thickness of 1 μm or less, it is too thin andhence the effect of the present invention decreases. It should be noted,however, that in the condition (1), even when the porous heat-resistantlayer has a thickness of 1 μm or less, relatively good results wereobtained.

As for the batteries 49 to 54, the cycle life characteristic wasremarkably low in the condition (2) regardless of the thickness of themain flat portions of side wall of the battery can. Also, thesebatteries were somewhat remarkably overheated on the nail penetrationtest when being charged to 4.45 V. This indicates that when the porousheat-resistant layer has a thickness of 20 μm or more, it is too thickand hence the effect of the present invention decreases.

As a general tendency, when the main flat portions of side wall of thebattery can are too thick (e.g., more than 1000 μm), the cycle lifecharacteristic was low in the condition (2), and when the main flatportions of side wall of the battery can are too thin (e.g., 80 μm), thebattery was significantly overheated on the nail penetration test whenbeing charged to 4.45 V.

The prismatic lithium secondary battery of the present invention has anexcellent short-circuit resistance, a high level of safety, andexcellent high-rate discharge characteristics. Therefore, it can be usedas a power source for any portable appliances, for example, personaldigital assistants and portable electronic appliances. The prismaticlithium secondary battery of the present invention can also be used as apower source for small-sized power storage devices for home use,two-wheel motor vehicles, electric vehicles, and hybrid electricvehicles, and its application is not particularly limited.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

1. A prismatic lithium secondary battery comprising: a prismatic batterycan having a bottom, a side wall, and an open top; an electrodeassembly; a non-aqueous electrolyte; and a sealing plate covering theopen top of said battery can that accommodates said electrode assemblyand said non-aqueous electrolyte, wherein said electrode assemblycomprises: a positive electrode having a positive electrode activematerial layer; a negative electrode having a negative electrode activematerial layer; and a porous heat-resistant layer and a separator thatare interposed between the positive and negative electrodes, said porousheat-resistant layer comprises an insulating filler and being in contactwith the active material layer of said positive or negative electrode,said side wall of the prismatic battery can has two rectangular mainflat portions that are opposed to each other, and a thickness A of saidporous heat-resistant layer is 2 to 10 μm, a thickness B of each of saidmain flat portions is 160 to 1000 μm, and A and B satisfy the relation:0.005≦A/B≦0.03.
 2. The prismatic lithium secondary battery in accordancewith claim 1, wherein said positive electrode is a strip-like positiveelectrode that comprises a positive electrode core member and thepositive electrode active material layer carried on each side of thepositive electrode core member, said negative electrode is a strip-likenegative electrode that comprises a negative electrode core member andthe negative electrode active material layer carried on each side of thenegative electrode core member, said strip-like positive electrode andsaid strip-like negative electrode are wound together with said porousheat-resistant layer and said separator interposed between the positiveand negative electrodes, and said porous heat-resistant layer is carriedon a surface of at least one of the two active material layers that areformed on both sides of the core member of at least one of the positiveelectrode and the negative electrode.
 3. The prismatic lithium secondarybattery in accordance with claim 1, wherein said insulating fillercomprises an inorganic oxide.
 4. The prismatic lithium secondary batteryin accordance with claim 1, wherein the porosity of the porousheat-resistant layer is 40 to 80%.
 5. The prismatic lithium secondarybattery in accordance with claim 1, wherein the porosity of the porousheat-resistant layer is 40 to 65%.